EVENTSDIAGNOSTICS Photo courtesy of Končar D&ST, Zagreb, Croatia Zagreb, D&ST, Končar of courtesy Photo

Chromatographic analysis of gases from the The process of creating and detecting gases in the

1. Introduction ABSTRACT Given the demands of today’s power systems, the question ince the beginning of production of oil for transformers of the transformer reliability is one of the top priorities. it has been shown that during their work, due to various The need for monitoring the performance of transformers Sphenomena, certain gases may appear [1]. In 1928, Buch- is imposed since the beginning of their use. This article holz relay was first used to collect gas bubbles passing through aims to analyse the impact of the gases released during the oil to the conservator. Due to the occurrence of gas in the breakdowns and energy discharge within the transformer transformer, the oil is gradually being displaced, which in a cer- that could accelerate the degradation of the insulation sys- tain amount pulls down the float and turns on a warning signal. tem and gradually lead to major faults and incidents. In larger amounts of gas, Buchholz relay trips the transformer.

Consequently, there was a need for a quality analysis of the situ- ation inside the transformer in order to detect and eliminate po- KEYWORDS tential failures. In the early sixties gas chromatograph appeared, transformer, analysis, oil, gas, insulation a device used to identify gases dissolved in . Since the seventies, this analysis is used to detect a number of different

36 TRANSFORMERS MAGAZINE | Volume 2, Issue 1 Emir ŠIŠIC

Chromatography is a separation me- thod in which components are sepa- rated and isolated in stationary and mobile phases

of materials (out of which many are forming gases in the oil and above), damage to the insulation, reduction of operational safety, and ultimately failure or breakdown.

Mineral transformer oil and cellulosic materials (paper) are usu- ally used for the transformer insulation. Cellulose insulation gives the transformer dielectric strength and spacing between windings as well as the space between windings which are at a different potential. Mineral transformer oil, due to the impreg- nation in paper, increases its dielectric strength.

The oil is mainly composed of saturated hydrocarbons linked by C-C and C-H bonds which crack as a result of faults with the formation of unstable parts, radicals or ions as well as other com- pounds. They are easily recombined in gas molecules, like in or- der by energy activation: hydrogen (H2), methane (CH4), ethane (C2H6), ethylene (C2H4), and acetylene (C2H2).

Causes for forming gases can be classified into three categories: corona or partial discharge, pyrolysis (decomposition of subs­ tances under the influence of high temperatures), thermal de­ composition and sparks. Most of the energy is released during sparks, followed by overheating, and lastly due to the appearance of the corona. Gases that appear during fault are typical for de- gases. Seven of them are characteristic for degradation of transfor- gradation of the insulation system: hydrocarbons, carbon oxides mer oil-paper insulation: hydrogen (H2), methane (CH4), ethane and gases which do not come from failures [3]. Mentioned gases (C2H6), acetylene (C2H2), ethylene (C2H4), carbon monoxide (CO) begin to form at certain temperatures as shown in Fig. 1. and carbon dioxide (CO2). In addition, some gases (C3Hy) can also appear but they do not affect the work of the transformer much.

Today chromatographic analysis of the transformer’s gases is one of the most important and the most sensitive methods for the early detection of changes in state of oil-paper insulation and thus repre- sents an excellent indicator of the existence of potential failure [2].

Chromatography (derived from the Greek words chroma - colour and grafein - write) is the collective name for a group of techniques used for separating mixtures. It is a physical separation method in which the tested components are separated and isolated in two phases: stationary (fixed) and mobile (movable). Chromato- graphy as a laboratory technique can be: analytical (works with small samples and attempts to assess the contribution of indivi- dual components in the mixture) and preparative (separation of components from the mixture and a kind of purification).

2. Occurrence of gases in the transformer

During the work transformers are exposed to electrical, thermal and mechanical stresses, leading to degradation of the insulation. Figure 1: The appearance of flammable gases at average temperature of The consequences are rapid chemical reactions and degradation oil decomposition www.transformers-magazine.com 37 DIAGNOSTICS

Following the ratio of gases, it is important to consider the so- Modern gas chromatographs consist lubility of gases as a function of temperature. Fig. 2 shows that over the temperature range from 0 to 80 °C, solubility of some of the gas supply unit, the sampling gases increases above 79 %, while the solubility of other decre- unit with automatic injector, the co- ases below 66 %. lumn and the oven. It is equipped with temperature controllers and microprocessor systems

Figure 3: Distribution between the two phases: A) equilibrium of molecules between stationary phases Figure 2: Solubility of gases as a function of temperature B) model of chromatographic process

3. Test methods tion of the two phases: fixed immobilised in a column or on a flat surface and movable, carrying the ingredients of the mixture Oil swabs are conditioned by constructional features of the trans- through the system. Different travel speed of individual compo- former. Well equipped devices have valves for sampling from nents will lead to separation (Fig. 3). In the initial phase the tested three levels of tank height as well as from the Buchholz relay and sample is introduced to a stream of inert carrier gas into a chro- the tank. It is important to emphasise that gases from matography column filled with solid or liquid stationary phase. the oil develop only when it is in operation, thus a quality repre- Liquid stationary phase may be absorbed on an inert solid sup- sentative sample should be taken while it is working, or soon af- port or can be immobilised on the walls of the capillary (capillary ter it has been shut down from service. column). The separation of mixtures will be carried out provided that they have different solubility in the liquid stationary phase. In case of sudden gas evolution, depending on the location and The first component that comes out is the one that is the least intensity of appearance, their distribution is uneven at first, but soluble in the stationary phase. equalises after a while. Therefore, in some cases it is necessary to take more samples of oil from different places immediately after Modern gas chromatographs consist of four units supported by the observed phenomena and again after a certain time. For con- temperature controllers and microprocessor systems [4]. The tinuous monitoring of results from the same transformer, it is im- first unit (gas supply unit) provides all the necessary gas supplies portant that samples are taken in the same way at the same place which may involve a number of different gases, depending on the and examined with the same test method. To examine and analy- chosen detector. Some detectors require hydrogen, air or oxygen se the gas composition, the sample must be taken from Buchholz to support combustion and a mobile phase supply that could be Relay or must be extracted from the oil. helium, nitrogen or some other inert gas. The second unit is the sampling unit with automatic injector with its own oven. The co- Testing is carried out using gas separation or chromatography. lumn represents the third unit, the important device that achieves Chromatography principle is based on equilibrium distribu- separation and an oven to control process temperature. Oven is used to maintain the temperature of the process at the optimum level and to improve separation of components. It should operate Gases from the oil develop only when over a wide temperature range (5 °C - 400 °C), but in practice the maximum oven temperature needed is usually less than 250 °C. the transformer is in operation, thus The oven often has air circulation driven by a powerful fan to en- a quality representative sample sure even temperature throughout the oven. The temperature in all parts should be stable at ± 0.5 °C.The fourth part of chromatograph should be taken while it is working, is the detector with wide range of available types. The output from or soon after it has been shut down the detector is usually electronically modified with data processing from service computer which processes the data and prints out report. After insertion of the components (mixed only with inert gas – mobile phase) and their separation at the exit of the column,

38 TRANSFORMERS MAGAZINE | Volume 2, Issue 1 they are placed in the detector. Detectors help visualising the Based on the results of gas analysis components eluted from the column. The obtained result after processing represents an input signal to the printer. The detector it is possible to diagnose three fun- registers a series of symmetric peak evaluations whose respon- damental causes of the insulation se depends on the concentration of ingredients in the sample which is recorded as a function of time. The position of the peak degradation which are divided into on time axis helps identifying the ingredients, and in areas un- six typical problems der the peaks, the exact amount of each separate ingredient can be calculated. Fig. 4 shows the output of the two component gas chromatographic analysis.

4. Interpretation of results Based on the results of gas analysis it is possible to diagnose three fun- damental causes of the insulation degradation which are divided into six typical problems [5] with following observable consequences:

(1) Partial discharge – PD [6] (2) Discharge with low energy – D1 (3) Discharge with high energy – D2 (4) Thermal fault (up to 300 °C) – T1 (5) Thermal fault (above 300 °C) – T2 (6) Thermal fault (above 700 °C) – T3

It often happens that one phenomenon steps into another or that two things occur simultaneously. Once we know the concentra- tions of certain gases, we can proceed with choosing the method by which they will be interpreted. Interpretive methods use several ratios of gases to qualify a few different states of the transformer.

The ratios of gases used by interpretive methods are:R 1=CH4/H2, Figure 4: Output of the chromatographic tests with 2 components R2=C2H2/C2H4, R3=C2H2/CH4, R4=C2H6/C2H2, R5=C2H4/C2H6, R7=C2H6/CH4. Gas concentration is expressed in ppm or µl/l [7]. Results of this method on a power transformer are shown in Fig. 5. Key information about the gases that appeared during the tests 4.1. Roger’s ratio method are shown in Table. 1. The flame ionisation detector (FID) is used as a detector. The sample gas is introduced into a hydrogen flame This method utilises four gas ratios:R 1, R2, R5, and R7. Combina- inside the detector. Any hydrocarbons in the sample will produce tion of the coding of these ratios gives 12 different types of trans- ions when they are burnt. The generation of these ions is propor- former faults (normal deterioration, partial discharge, overheating tional to the concentration of organic species in the sample gas (<150 °C, 150 °C - 200 °C, 200 °C - 300 °C), general conductor over- stream. heating, winding circulating currents, core and tank circulating currents, flashover without power follow through, arc with power follow through, continuous sparking to floating potential and par- tial discharge with tracking. Roger’s ratio method provides a wide range of results, but the consistency of the method is quite low.

4.2. IEC method

IEC method originated from the Roger’s ratio method, except Figure 5: Graphic result of gas chromatographic analysis that the ratio R7 is excluded if concentration of gases is above ty-

Table 1: Key information about gases that appeared during test

No. Time [min] Height [µV] Area [min*µV] Name Concentration 1 3,60 129222 18988 CO2 6853 ppm 2 4,47 26792 2712 C2H4 468 ppm 3 4,90 109098 23541 C2H2 4580 ppm 4 9,85 12742 1149 CH4 329 ppm 5 10,37 14009 1278 CO 372 ppm

www.transformers-magazine.com 39 DIAGNOSTICS

The problem in the monitored trans- thod, done after chromatographic analysis. As seen in Fig. 7 the problem is registered in the area of low energy discharges. Today, former was identified as low energy Duval triangle method is one of the most accurate ones to detect discharges by the Duval triangle me- early failures in the transformers with best results for correct pre- thod dictions.

pical values [6]. Acceptable values of gases in normal operation of a transformer are: (C2H2 without tap (2-20), with tap (60-280), H2 (50-150), CH4 (30-130), C2H4 (60-280), C2H6 (20-90), CO (400-600), and CO2 (3800-14000) (ppm)).

4.3. Doernenburg ratio method

This method utilises the gas concentration from the following ratios: R1, R2, R3, and R5. In this method the gases must be pre- sent in significant amounts in order to guarantee proper results: (H2 > 100, CH4 > 150, CO > 350, C2H2 > 35, C2H4 > 50, and C2H6 > 65). This method, being one of the oldest ones, is the least accurate and a significant disadvantage is the need for greater amount of gas to make it usable.

4.4. Duval triangle method

Mr. Duval developed this method in the 1960s. Three types of hydrocarbons are monitored: methane, ethylene and acetylene. This method classifies six types of errors [8]. If the coordinates are located in the PD field, it indicates that partial discharge oc- Figure 7: Interpretation of the result with Duval triangle method curred, as shown in Fig. 6.

4.5 Key gas method

Key gas method could present damage levels of the transformer and its cause by analysing the levels of combustible gases. The method defines the level of damage by taking into considerati- on all of the total combustible gases which can be classified in different ranges. The presence of the fault gases depends on the temperature or energy that will break the links of the insulation or oil chemical structure. This method uses the amount of the individual gases rather than gas ratios for fault detection. Prin- cipal gases for general faults types are: ethylene – overheated oil, carbon monoxide – overheated cellulose, hydrogen – corona in oil, acetylene – arcing in oil. Key gas method is very reliable and provides good results in the initial detection of problems within the transformer.

4.6 Nomograph method

Figure 6: Duval triangle method Nomograph method combines the fault gas ratio concept with the Key gas threshold value in order to improve the accuracy Boundaries within the triangle are defined on the basis of expe- of fault diagnosis. It has to provide both graphic presentation rience, from a large number of failures that have occurred in the of fault-gas data and the means to interpret its significance. The transformer worldwide during the last sixty years. The problem nomograph consists of series of vertical logarithmic scales repre- in the transformer from Chapter 3 was discovered by this me- senting the concentrations of the individual gases [9].

40 TRANSFORMERS MAGAZINE | Volume 2, Issue 1 5. Conclusion [5] IEEE C57-104-1991, IEEE guide for the interpretation of gases generated in oil-immersed transformers, IEEE 1991 It is very important to know the condition of the transformer, [6] IEC 60599, Guide to the interpretation of dissolved and free not only during the tests, but also in operating mode. With the gases analysis, NTT IEC 1999 methods of analysis (chromatography) of gases from the trans- [7] and the Duval triangle, TechCon former, it is possible to quickly and easily get to the key results Asia Pacific, 2006 that will show partially the condition of the transformer. In the [8] IEC Publication 60599, Mineral oil-impregnated electrical industry, due to dirt and very common failures and electrical equipment in service - Guide to the interpretation of dissolved stresses, it is necessary to arrange such tests and analysis of oil and free gases analysis, March 1999 at least once a year. Timely detected faults are remedied easier, [9] A nomograph for interpretation of LTC-DGA data, NETA faster and cheaper, thanks to a lesser extent of the damage and the Conference, Jakob F., Jakob K., Jones S., Youngblood R., March 2004 possibility of repairing certain faults on the spot, which elimina- tes the difficulties associated with transportation of transformers (especially large units) for repair. Further, the repairs can be plan- ned and aligned with the needs of power system and consumers. Author Emir ŠIŠIĆ, holds an Electrical enginee- References ring degree from the University of Tuzla, Bosnia and Herzegovina. Emir started his [1] Gas chromatography, R.P.W. Scott, 2003. career at ArcelorMittal Zenica. He is now [2] Basic gas chromatography, H.M. McNair, J.M. Miller, July 2009 Manager of Electric distribution section in [3] Interpretation of chromatographic analysis of the trans­ Energy department. He has participated former’s­ gases, S. Cabrajac, A. Hadzi-Skerlev, CIGRE technical in several projects aimed at improving the brochure, October 1999 power system and related to increasing the power factor, the se- [4] Application of dissolved gas analysis method for power lectivity of power protection and reliability of the transformer transformer with the on load tap changer, N. Cincar, Z. Stojko- working conditions. vic, A. Simovic, S. Jokic, March 2011

www.transformers-magazine.com 41